1. Field of the Invention
The present invention relates to a hydraulic control apparatus of an automatic transmission mounted on a motor vehicle, and more particularly to a technique applied to the automatic transmission having a plurality of transmission gear trains.
The present application claims priority from Japanese Patent Application No. 2001-180976, the disclosure of which is incorporated herein by reference for all purposes.
2. Description of the Related Art
A manual transmission (MT) executing a gear shift operation by a manual operation of a driver includes an input shaft connected to an engine and to which a plurality of drive gears are attached, and an output shaft connected to a drive wheel and to which a plurality of driven gears forming pairs together with the drive gears are attached, in which a plurality of transmission gear trains are provided between the input and output shafts. In the MT, a gear shift operation, i.e., a gear change is performed by manually switching a switching mechanism such as a synchromeshed mechanism in order to switch a gear train, that is, a gear pair among a plurality of the transmission gear trains after disengaging a clutch at a time of the gear change, thereafter connecting the clutch.
When the gear change and clutch operation are performed by using a shift actuator actuated by an oil pressure, an automatic transmission can be obtained whose structure is based on a manual transmission. An automatic transmission of a type having a plurality of transmission gear trains (Automated Manual Transmission, hereinafter referred to as an AMT) has some advantages in comparison with an ordinary torque converter type automatic transmission (AT) having a planetary gear in an automatic gear change mechanism. Namely, it is possible to reduce the number of parts, thereby lightening it, and the transmitting efficiency of a drive system is higher than that of the automatic transmission of the ordinary torque converter type.
For example, Japanese Patent Application Laid Open No. 2000-55184 discloses such an AMT-type automatic transmission which has a starting clutch, that is, an input clutch for switching over from an engaged state to disengaged state and vice versa between a crank shaft and an input shaft of the engine, and a bypass clutch of a hydraulic multi-disc type for preventing a torque disconnection when transmitting a torque from the input shaft to the output shaft at the time of a gear shift operation.
The AMT disclosed in this publication performs such a fail safe system that a first gear stage is forcibly setted by a return spring when an oil pump constituting an oil pressure source fails. In the conventional fail safe system, however, when a solenoid valve becomes uncontrollable as a result of an electrical system failure, a gear change to a first gear stage is forcibly made even while a vehicle is traveling in a fifth gear stage with a wet multi-disc clutch engaged, and thus it results in that not only a sudden drop in speed is caused and thereby the vehicle become unstable, but also such problems as wear or scorching of the starting clutch occurs in case of a half clutch position. Furthermore, as the conventional fail safe system is directed only for a forward travel, the fail safe system in a reverse travel has not ever been considered to be applied.
An object of the present invention is to enable a gear shift to a transmission gear train used for backward moving to be reliably performed without gear clashes or interlocking when a reverse range is selected by a manual operation, even if an electrical system failure occurs in an automatic transmission having a plurality of transmission gear trains.
A hydraulic control circuit of the present invention has an input shaft on which a plurality of drive gears are provided, an output shaft on which a plurality of driven gears are provided to form the transmission gear trains by meshing with the drive gears, and a plurality of switching mechanisms for switching transmission gear trains transmitting drive power from the input shaft to the output shaft. As features of the present invention, it comprises an input clutch for engaging and disengaging between an engine and the input shaft, a select actuator for selecting either one of the plurality of switching mechanisms to perform a switching operation, a shift actuator for performing the switching operation of the selected switching mechanism, and a select position detection valve for opening and closing an oil passage under an engagement with the select actuator. It further comprises a shift position detection valve for switching the oil passages under an engagement with the shift actuator, the shift position detection valve supplying oil pressure for switching into a reverse gear stage to the select actuator when the shift actuator is actuated to a neutral position, and a forcible reverse shift valve for setting the shift actuator at a reverse gear stage by the oil pressure from the select position detection valve when the select actuator is switched to a reverse position. Thus, the oil pressure is supplied to the input clutch via the shift position detection valve when the reverse gear stage is setted by the shift actuator.
The hydraulic control circuit of the present invention further comprises a normally open solenoid valve for controlling a supply of the oil pressure to allow the select actuator to be actuated to the reverse position, and
a normally closed solenoid valve for controlling supply of the oil pressure to allow the select actuator to be actuated to a forward travel position.
According to the present invention, in the case that the selector lever is setted at the reverse gear stage, it is possible to actuate the select actuator to the reverse position after the shift actuator is actuated to the neutral position, and then to engage the input clutch after the shift actuator is actuated to the reverse position, thereby enabling switching to a reverse gear stage to be reliably performed without the gear clashes or interlocking.
Even in a case where a solenoid valve for switching the transmission gear train does not operate due to the electrical system failure, it is possible to actuate the select actuator to the reverse position after the shift actuator is actuated to the neutral position, and then to engage the input clutch after the shift actuator is actuated to the reverse position only by a manual operation, that is, by only setting the select lever at the reverse gear stage, thereby enabling the switching to the reverse gear stage to be reliably performed without the gear clashes or interlocking.